In the words of the great Thomas Petty, the waiting is indeed the hardest part.https://www.youtube.com/watch?v=uMyCa35_mOg
I've been waiting for the Sigma 24-105 for A-mount since they announced the lenses together in, I want to say, October-ish 2013. I saw a non-functioning Canon model of the lens at Photoplus 2013. Exactly a year later, I saw a functioning Canon model at Photoplus 2014 but no Nikon model and the Sony model was even up in the air. Sony has a gigantic hole in the A-mount lineup in that there is no serviceable modern 24-105-ish range lens with ultrasonic motor despite the loud demand for such a lens. Sigma would have made bank if they had managed to ship it on time, but they're only getting around to shipping it at the end of this June. That lens was vapor for 20 months. I was ready to buy it as soon as they would ship it.
Now, Sony is supposedly releasing a 24-105G this fall, and if they do, Sigma isn't getting my money, even if their 24-105 will be cheaper by $400 or so. I'm getting by with older Minolta lenses for the time being on a99, but Tamron is able to ship their Sony mount versions at about the same time as the other mounts, if not a month later.
It would not be the first time Sigma has jerked us around, they specifically announced A-mount versions of the newer 120-300 f/2.8 and then nine months later just up and said "nah, we're not doing it anymore." The guy at the Sigma booth at PPE2014 said "Well, if you preorder" and I'm like "No, I'm not preordering without some kind of target date."
To Sony's credit, with the exception of the 500 f/4, when Sony has announced products with actual ship dates, they generally adhere to those dates. I ordered the RX100 mk1 sight unseen; it shipped exactly a month after it was announced and it has been worth every cent.